1. Field of the Invention
The present invention relates to a method of processing a base plate for a magnetic disc, and, more particularly to a method of processing a surface of a base plate for a magnetic disc of a high record density, e.g., a magnetic disc having a plated or sputtered magnetic film, thereby improving lubricity and durability of the disc.
2. Description of the Prior Art
In general, a magnetic disc coated with a magnetic thin film must withstand abrasive forces and shocks caused by contact with a magnetic head when the rotational motion of the disc is started or ceased. Usually, therefore, a SiO.sub.2 film is provided so as to cover and protect the magnetic film formed on the base disc, and a liquid or solid lubricant is applied to the protective film. It is preferred to use a base plate having an A1 alloy surface, a plated metal surface or an anodized A1 alloy surface, depending on the magnetic medium provided thereon. The surface of the base plate is processed so as to have a predetermined smoothness, before a magnetic film is formed thereover.
When a Ni-P surface plated on a base plate of an A1 alloy is polished with a polishing cloth and a polishing solution of colloidal silica containing ultra-fine particles of silica, the surface is finished to have a very small unevenness of about 50 .ANG. in height. A magnetic disc may be completed by providing a SiO.sub.2 protective film over a magnetic film formed on the finished base disc and also applying a liquid lubricant to the protective film. With the magnetic disc thus completed, the liquid lubricant and water droplets condensed from air spread thin between the disc surface and a magnetic head when the magnetic head rests on the magnetic disc. As a result, the magnetic disc and head become sticky to each other due to surface tension of the thin liquid therebetween and then it becomes hard to initiate rotation of the magnetic disc. Further, since the surface of the magnetic disc is so flat the coefficient of friction between the magnetic disc and head becomes large, and scratches then tend to be caused on the disc surface.
In view of the above described problems, the Japanese Patent Laying-Open Gazette No. 61-29418 (1986) discloses a method for intentionally realizing a prescribed fine unevenness over a magnetic disc surface. In that method, a lapping film is brought into contact with a given pressure with a surface of a base disc in a rotational state, Abrasive grains on the lapping film causes a fine unevenness over the base disc surface. However, in the event that the abrasive grains aggregate to larger particles, significant scratches are sometimes caused on the base disc surface, and it is also likely that dust is caused by separation of the abrasive grains from the lapping film and abrasion of the base disc surface.